The application of a coating by means of electrodeposition is known. Electrodeposition is a known coating method which comprises dipping an electrically conductive material to be coated into a suspension of a charged film-forming material dispersed in water, and subjecting the conductive material to electrocoagulation by the passage of electric current through the suspension, and carrying out a baking treatment of the conductive material coated with the electrocoagulation.
Electrodeposition has major advantages in that the loss of coating material is low; the process is easily automated and controlled thereby reducing labor costs; a variety of materials to be coated can be treated simultaneously; uniform film formation is possible in the inside and edge of the materials to be coated; and coating materials have good adhesiveness to the materials to be coated. Waterborne coating materials have an improved ecological profile and are preferred in view of environmental pollution and disaster prevention.
The actual coating process usually involves submerging the part to be coated into a container which holds the coating bath or solution and applying direct current electricity through the bath using electrodes. Typically voltages of 25-400 volts DC are used. The object to be coated is one of the electrodes, and a set of “counter-electrodes” are used to complete the circuit.
After deposition, the object is normally rinsed to remove the undeposited bath. The rinse is followed by a baking or curing process. This will crosslink the polymer and allows the coating to flow out and become smooth and continuous.
The process is useful for applying materials to any electrically conductive surface. Consequently cationic electrodeposition is widely employed as coating method in the car industry as a coating method for car bodies. However for heat-sensitive materials such as metalised plastics, coating compositions with a thermosetting temperature above 100° C. cannot be used.
In order to solve this problem, there is a method for coating materials using ultraviolet radiation. This method employs coating compositions with radiation curable, preferably UV curable or electron beam curable, oligomers, monomers, photo polymerization initiators, radical inhibitors. In these solventless coating materials, the reactive monomer is used for diluting other components in place of an organic solvent and thus becomes part of the cross-linked polymer network after radical polymerization. Cured coating film produced from this type of coating material is very hard, but fragile and less adhesive to the base material, which is particularly problematic on a smooth surfaced base material.
Coating compositions have been described which contain both electrodepositable and radiation curable properties. A problem however is that the energy curable part has poor solubility in water. It is therefore problematic to obtain a composition with good dispersibility.
U.S. Pat. No. 6,232,364 discloses the use of two or more carefully selected species of photo initiators in order to prevent this phenomenon. However this selection process is tedious. A more robust and less critical method is sought for.
There remains a need in the art for improved coating compositions, especially for heat sensitive materials, and for their manufacturing processes.
The present invention aims to resolve at least some of the problems mentioned above. The invention thereto aims to provide coating compositions for electrodeposition and radiation curing with high reactivity, low tackiness, good adhesion, high anticorrosion, and high gloss. It is a further object of the invention that the coating compositions are suitable for coating heat-sensitive materials such as plastics.